Lecture Notes: Pulmonary Circulation, Ventilation, and Perfusion

Key Concepts

• Ventilation (V): The amount of air reaching the alveoli for gas exchange.
• Perfusion (Q): The blood flow reaching the alveoli ready for gas exchange.
• Ventilation-Perfusion Ratio (V/Q ratio): Important for understanding how well lungs are oxygenating blood and removing CO2.

Lung Zones and V/Q Ratio

• Three Zones:
  • Zone 1 (Apex):
    • Least perfusion due to gravity.
    • Larger alveoli with reduced blood flow.
    • Results in 'wasted ventilation' as there is insufficient perfusion for gas exchange.
    • V/Q Ratio is high due to less perfusion.
  • Zone 2 (Middle):
    • Balanced perfusion and ventilation.
    • Good gas exchange.
  • Zone 3 (Base):
    • Most perfusion and ventilation due to gravity.
    • Smaller alveoli expand more effectively.
    • V/Q Ratio is low as perfusion increases more than ventilation.

Factors Affecting V/Q Ratio

• Gravity: Primary factor causing differences in V/Q ratio in different lung zones.
• Diseases: Can significantly alter V/Q ratios.

Understanding V/Q Ratio Changes

• Normal Average V/Q Ratio: 0.8, indicating more perfusion than ventilation.
• Pathological Changes:
  • Pneumonia, COPD, Pulmonary Edema: Decreased ventilation, thus lower V/Q ratio.
  • Pulmonary Embolism: Causes increased V/Q ratio due to reduced perfusion.
  • COPD: Mucus build-up reduces ventilation; destruction of capillaries reduces perfusion, especially in late stages.

Physiological Concepts

• Physiological Dead Space: Occurs when there is ventilation without perfusion.
• Physiological Shunt: Occurs when there is perfusion without ventilation.

Special Considerations

• Waterfall Effect: Height of lung zones does not affect flow.
• Absolute Dead Space: Where gas enters alveoli but does not participate in gas exchange, e.g., trachea.

Summary

• Ventilation and Perfusion: Both increase from the apex to the base of the lung.
• V/Q Ratio: Decreases from the apex to the base, influenced by gravitational effects.
• Clinical Correlation: Understanding V/Q ratio helps in diagnosing and managing pulmonary diseases.